Miranda, one of 28 icy moons around Uranus, may have had a subsurface ocean, challenging what planetary scientists previously thought about the distant satellite.
The find came just as NASA revealed that data from Voyager 2, our only close-up view of distant Uranus, may not have been interpreted correctly.
Voyager 2 flew by the planet in 1986, imaging several of Uranus's moons.
Its photos of Miranda showed a patchwork of grooved terrain and craters.
A new study examined what geological processes could produce this appearance.
It found the most likely scenario is that 100–500 million years ago, Miranda had a 100km-deep (62-mile) ocean beneath its ice shell.
Miranda has a radius of just 235km (146 miles), meaning this ocean would take up most of the interior.
Even now, the moon could still have the remnant of a thin ocean layer.
"To find evidence of an ocean inside a small object like Miranda is incredibly surprising," says Tom Nordheim from Johns Hopkins Applied Physics Laboratory, who took part in the study.
"There may be several ocean worlds around one of the most distant planets in our Solar System, which is both exciting and bizarre."
At the same time, NASA has revealed that it might be time to rethink some of Voyager 2’s measurements of Uranus.
During the craft’s fly-by, it found the magnetosphere – the magnetic bubble around the planet – curiously devoid of water ions.
This seemed to suggest the moons were all frozen solid and therefore not releasing any water.
However, a recent report revealed that Voyager 2 passed Uranus during an extreme space weather event that would have squashed the magnetosphere, reducing the number of ions the spacecraft saw.
"If Voyager 2 had arrived just a few days earlier, it would have observed a completely different magnetosphere at Uranus," says Jamie Jasinski from NASA’s Jet Propulsion Laboratory, who led the report.